Physik: DPG Tagungen - Sitzung O 28 - Nano-science.de

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Physik: DPG Tagungen - Sitzung O 28 http://www.dpg-tagungen.de/archive/2004/o_28.html Wasserstoff-Brückenbindungen sind. Bei gleicher Auslenkung sind die Rückstellkräfte parallel zur Wasserstoff-Brückenbindung größer als senkrecht zu ihr. Mit Hilfe von LDOS-Bildern, die aus DFT-Rechnungen erstellt wurden, wird am Beispiel der Selbstassemblierung von Adenin auf Graphit versucht die experimentell gewonnenen Bilder einer Drehserie zu simulieren. O 28.10 Poster Mi 16:00 Bereich C Elctronic structure of MgSO 4 crystals •V.V. Maslyuk 1,2 , C. Tegenkamp 1 , T. Bredow 3 und H. Pfnür 1 1 Institut für Festkörperphysik, Appelstr.2 D-30167 Hannover, Germany 2 Uzhhorod National University, Pidgirna St.2 88000 Uzhhorod, Ukraine 3 Institut für Theoretische Chemie, Am Kleinen Felde 30 D-30167 Hannover, Germany Typically, the crystallographic structures of minerals are quite well investigated, but there is not much knowledge about their electronic structures. As one candidate of an insulator consisting chemically of more than two different elements, we investigated the crystallographic and electronic structure of MgSO 4 and kieserite (MgSO 4 ·H 2 O), respectively. Starting with kieserite with 36 atoms per unit cell, we determined the electronic structure by ab-initio calculations. Using CRYSTAL03 the lattice parameters were calculated within an accuracy better than 0.01 Å by choosing basis sets for magnesium and oxygen taken from MgO and for sulfur taken from CaSO 4 including also d-orbitals. In order to calculate the band gap of kieserite, the method of hybrid mixing of Hartree-Fock and DFT hamiltonians was used. The band gap at the Γ-point was detemined to be 7.4 eV. To support the theoretical calculations, first photoelectron- and electron energy loss spectroscopy experiments have been performed. Thereby, bulk kieserite crystals have been used. Obviously, the effect of surface relaxation effects and defects are small, because the measured band gap agrees quite well with the theoretical findings. Nevertheless, our calculations will be further refined. O 28.11 Poster Mi 16:00 Bereich C Valence band photoemission spectroscopy of chemically disordered ultrathin Ni x Pd 1-x films on Cu 3 Au(100) •Frank Matthes 1 , Liu-Niu Tong 1 , Alexej Rzhevskii 2 und C.M. Schneider 2 1 Leibniz Institut für Festkörper- und Werkstoffforschung Dresden, PF 27 01 16, D-01171 Dresden 2 Institut für Elektronische Eigenschaften, Forschungszentrum Jülich, 52425 Jülich We have studied the valence band structure of ultrathin Ni x Pd 1-x alloys on a single crystalline Cu3Au(100) substrate. Ni and Pd are isoelectronic, but while Ni is ferromagnetic with an average exchange splitting of 0.3 eV along the ∆ direction, Pd exhibits a higher spin orbit coupling (band splitting 0.3 eV). Via the alloy concentration one can conveniently vary the relative weight of these two spin-dependent interactions with profound effects for the electronic structure. We will present first results of our high-resolution photoemission measurements for 15 monolayer thick Ni x Pd 1-x films with different compositions. The measurements were performed along the ∆ direction for excitation energies from 16 eV to 40 eV. As a main spectral feature, we observed contributions from bands originating from 3d Ni and 4d Pd electrons near the Fermi energy. This feature only disperses weakly with the excitation energy. Starting with pure Ni films, we observed a broadening of the spectra while increasing the Pd concentration up to 30%. This may be caused by the chemically induced disorder. To distinguish the spin dependent contributions and thus arrive a more detailed interpretation of the photoemission data, we performed for three different Ni rich alloy compositions also spin-resolved photoemission experiments. O 28.12 Poster Mi 16:00 Bereich C Site-specific Valence-electronic Structure of SrTiO 3 by X-ray Standing Waves and XPS 4 of 32 07.06.2009 22:14
Physik: DPG Tagungen - Sitzung O 28 http://www.dpg-tagungen.de/archive/2004/o_28.html •Sebastian Thiess 1 , Tien-Lin Lee 1 , Bruce C. C. Cowie 1 , Joe C. Woicik 2 und Jörg Zegenhagen 1 1 ESRF, Grenoble, France 2 NIST, Gaithersburg, USA We used the x-ray standing waves (XSW) technique and x-ray photoelectron spectroscopy (XPS) to determine the contribution of the constituent elements Sr, Ti and O of a SrTiO 3 (STO) crystal to the STO valence band, thus deriving their partial density of states. Lattice site-specific electronic information - which is not available from standard XPS - is obtained by utilising the spatial intensity modulation of an x-ray standing wave (XSW) interference field, generated by the coherent superposition of an incident and a Bragg-reflected x-ray beam. By proper positioning the antinodes of the XSW within the STO unit cell, photoemission from specific lattice sites can be preferentially excited and their valence electronic contribution identified. XSW fields were generated by the STO(111) and STO(112) reflections at photon energies of 2.7 and 3.9 keV. XSW modulated, high-resolution core and valence-electron emission spectra were recorded from an in-situ UHV annealed STO single crystal at beamline ID32 at the ESRF. O 28.13 Poster Mi 16:00 Bereich C Mechanisms and capability of Steam Laser Cleaning •F. Lang 1 , M. Mosbacher 1 , S. Georgiou 2 und P. Leiderer 1 1 University of Konstanz, Fach M676, 78457 Konstanz, Germany 2 FORTH - IESL, P.O. Box 1527, 71110 Heraklion, Greece Steam Laser Cleaning is a powerful approach to remove submicron particles from surfaces. In the first step a liquid is applied onto the contaminated surface. Afterwards a laser pulse heats part of the fluid to a superheated state resulting in bubble nucleation and growth. Thereby strong forces are exerted on the contaminants, which are able to overcome the adhesion of the particles to the surface. Our measurements covering a broad range of particle sizes demonstrated the existence of a universal cleaning threshold for the laser fluence. In addition the influence of different liquid layer thicknesses was explored. Investigations of the surfaces after cleaning verified for the first time that damage free removal can be achieved, if the experimental parameters are chosen properly. Moreover fundamental studies of the bubble nucleation and growth in the superheated liquid were conducted to gain a more detailed understanding of the phase transition dynamics. O 28.14 Poster Mi 16:00 Bereich C In-situ Video-STM Untersuchung der Dynamik von Sulfidadsorbaten auf Cu(100)-Elektroden •Tunay Tansel und Olaf Magnussen Institut für Experimentelle und Angewandte Physik Christian-Albrechts-Universität Kiel Die Diffusion und Wechselwirkung von Adsorbaten an Elektrodenoberflächen ist von großer Bedeutung für das Verständnis der elektrochemischen Phasengrenze. Solche dynamischen Prozesse wurden hier am Beispiel von Sulfidadsorbaten (Bedeckungen 0,01 bis 0,03ML) auf Cu(100) in 0,01M HCl mittels elektrochemischer Hochgeschwindigkeits- Rastertunnelmikroskopie (Video-STM) direkt auf der atomaren Skala untersucht. Eine statistische Analyse der Diffusion isolierter S ad ergab Sprungfrequenzen im Bereich von ? 2·10 -1 Hz. An den Domänengrenzen c(2x2)-Phase ist die S ad Mobilität jedoch stark erhöht. Dies zeigt, dass die Gegenwart des c(2x2)-Cl Koadsorbatgitters die effektive Diffusionsbarriere von S ad deutlich vergrößert. Weiterhin weisen die Beobachtungen auf attraktive S ad -S ad Wechselwirkungen und eine erhöhte Mobilität von Clustern an mehreren, benachbarten S ad hin. 5 of 32 07.06.2009 22:14
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